Graphene is a two-dimensional carbon crystal, with a hexagonal honeycombed structure.
This crystal has very surprising properties, making the material very popular with many public and private laboratories at the present time.
Indeed, firstly, graphene displays exceptional electron mobility at ambient temperature, of up to 20 m2/(V·s). By way of comparison, the electron mobility of indium antimonide InSb is, at ambient temperature, two and a half times lower, that of silicon is thirteen times lower, whereas that of silver is two thousand five hundred times lower.
Furthermore, graphene is transparent since the thickness thereof, which is that of a carbon atom, is in the region of 1 Angstrom (0.1 nm). Moreover, it offers a combination of light weight, high chemical stability and high mechanical strength (typically two hundred times greater than that of steel).
The prior art includes a number of techniques for producing graphene sheets such as micromechanical graphite crystal exfoliation, electrochemical reduction of graphene oxide, opening carbon nanotubes, catalytic growth of graphene on a metal substrate or silicon carbide sublimation.
Of these techniques, catalytic growth of graphene on a metal layer appears to be one of the most promising processes for producing graphene on an industrial scale and enabling the integration thereof in micro- or nanoelectro-mechanical or other devices, meeting industrial specifications.
Indeed, using this technique, it has been possible to produce graphene samples of high crystalline quality, characterized by monocrystalline ranges over several hundreds on microns and by a unique crystallographic orientation on a macroscopic scale. Graphene samples with centimetric surface areas have even been obtained, which is not the case of the other techniques for manufacturing graphene available to date.
In the initial version thereof, the catalytic growth of graphene on a metal layer is based on the exposure of a layer of a metal, for example nickel, copper or iridium, heated to a high temperature, i.e. approximately 1000° C., to gas-phase carbon species. Depending on the solubility of carbon in the metal forming the metal layer and the capacity thereof to diffuse in the metal, the formation of a graphene sheet with one or a plurality of graphene layers is obtained on the metal layer.
In this way, monolayer and bilayer graphene sheets on a nickel layer (Reina et al., Nano Res 2009, 2, 509-516, [1]), and monolayer graphene sheets on a copper layer (Li et al., Science 2009, 47, 2026-2031, [2]) and on a platinum monocrystal (Sutter et al., Phys. Rev. B 2009, 80, 245411, [3]) have been obtained.
The metal layer may then be removed by etching and the graphene separated from the layer transferred to another layer.
More recently, Hofrichter et al. (Nano Lett. 2010, 10(1), 36-42, [4]) demonstrates that it is possible to product “polygraphene”, i.e. a graphene sheet comprising areas with a single graphene layer and areas with a few graphene layers, on a nickel layer no longer using gas-phase carbon species but a solid carbon source, in this instance, a layer of amorphous silicon carbide.
Moreover, the benefit of thin layers of platinum silicides is known.
The semiconductor properties of these silicides are routinely used in the fields of radio-astronomy and electronics, particular for creating contact zones between silicon and one or a plurality of stacked metal layers, enhancing ohmic contacts in MOS (Metal Oxide Semiconductors) and CMOS (Complementary Metal Oxide Semiconductors) technologies, and producing Schottky diodes. The importance of platinum silicides in designing infrared detectors and heat cameras for medical imaging or implementing nano-lithographic methods should be noted.
As such, it is clear that it would be advantageous to have structures comprising a graphene sheet on a thin layer of platinum silicide, in order to combine the qualities of graphene with those of this type of silicide.
It would appear that, to the inventor's knowledge, no prior art document describes the synthesis of a graphene sheet on a platinum silicide. However, a process for producing a monolayer graphene sheet on a nickel silicide was recently described in the literature (Juang et al., Carbon 2009, 47, 2026-2031, [5]).
In this reference, the layer of silicon carbide is deposited on a silicon substrate, and coated with the nickel layer and the structure formed undergoes a heat treatment enabling the carbon to dissolve in the nickel, thus forming a layer of nickel silicide, situated at the interface of the layers of silicon carbide and nickel. The formation of the silicon carbide layer competes with a carbon migration phenomenon toward the surface of the nickel layer, the whole helping produce, by catalytic growth, a graphene sheet on the surface.
In principle, this method does not appear to be applicable to the synthesis of a graphene sheet on a platinum silicide, since it is based on the solubility and diffusion properties of carbon in the metal and, in this case, carbon is very slightly soluble in platinum.
The inventor thus set out to achieve the general aim of providing a method for producing a graphene sheet on a platinum silicide.
The inventor also set out to achieve the aim of the method making it possible to obtain a multilayer graphene sheet or monolayer graphene sheet.
Moreover, the inventor also set out to achieve the aim of enabling the synthesis on a platinum silicide in the form of spaced blocks or on a platinum silicide in the form of a layer and, in the latter case, giving rise to a graphene sheet wherein the surface, i.e. the area, is merely limited by the dimensions of the platinum silicide layer, regardless of the dimensions of the layer.
Finally, the inventor set out to achieve the aim of obtaining rapid synthesis, merely requiring moderate heat treatments.